Nowadays, LCD has been applied to many fields of applications such as cellular phones, digital still cameras (DSCs), digital video cams (DVCs), camcorders, global positioning systems (GPSs), personal digital assistances (PDAs), personal computers (PCs), notebooks, and televisions (TVs). LCD has been playing an important role and helps developing trends in the future with far more characteristics and advantages like thinness, low energy consumption, high resolution, no radiation, etc. Among many kinds of LCDs, TFT-LCD is so far the most popular planar display. Please refer to FIG. 1(a) which is a circuit diagram showing one of the pixel cells of a thin film transistor liquid crystal display. The state (ON or OFF) of the thin film transistor 11 is controlled by the scan-line voltage Vs. The data-line voltage Vd is applied to the liquid crystal layer 12 interposed between the pixel electrode 131 and the common electrode 132 when the thin film transistor 11 is in an ON state. The liquid crystal molecules in the liquid crystal layer 12 will align in predetermined directions according to the applied electric field, thereby adjusting the transmittance of the crystal liquid layer 12. The amount of light emitted from the light source 14 and passing through the liquid crystal layer 12 to reach the observer's eyes is controlled in this way to achieve the display effect.
The storage capacitor 15 is employed to enhance the electric characteristics of the liquid crystal display unit by compensating the charge storage capacity of the liquid crystal display unit. Otherwise, the voltage of the liquid crystal display unit will drop stage by stage due to current leakage after turning off the thin film transistor 11. It is known that the capacitance of the storage capacitor 15 will affect the display quality. For example, undesired flicker or cross-talk results from insufficient capacitance of the storage capacitor 15. The capacitance of the storage capacitor 15 is an essential consideration for designing the liquid crystal display unit.
Please refer to FIG. 1(b) which is a schematic diagram showing the structure of the conventional thin film transistor liquid crystal display. The thin film transistor structure 21 and the storage capacitor structure 22 are formed on the substrate 20. The storage capacitor structure 22 mainly consists of a bottom electrode 221, an insulator 222, and a top electrode 223. The bottom electrode 221 and the top electrode 223 are respectively made of polysilicon and metal in the low temperature polysilicon thin film transistor (LTPS-TFT) process. The most popular method for increasing the capacitance of the storage capacitor structure 22 is to enlarge the area of the electrodes 221 and 223. Increasing the length A, however, will decrease the size of the transparent area 23. Hence, the brightness of the display is adversely affected. For overcoming such problem due to increased size of the electrodes 221 and 223, a high-power light source 14 is needed. Without doubt, it wastes energy and does not conform to the economic interests. A better method and structure are desirable according to the present invention.